1. Field of the Invention
The invention relates to an acoustical device for the reception of sound by humans.
2. Description of the Prior Art
In some prior art headphone and earphone designs, it is necessary to employ either electrical or acoustical means to attain approximate reproduction of sound. An acoustic passage for the acoustical channeling of sound from one ear to the opposite ear is described in U.S. Pat. No. 3,939,310. Other prior art relative to stereophonic reception of sound by means of headphones or earphones includes U.S. Pat. Nos. 3,924,072; 3,863,028; 3,792,754; 3,098,307 and Re 25,652.
In our everyday life of listening to sounds, several things which we take for granted, enable us to locate and detect via ears and brain what takes place around us by way of sound. One consideration is binaural localization. We are able to determine, to a fair degree of precision, sound direction and often estimate even distance of sound sources. There are several factors which determine direction of arrival by sounds. One is intensity of arrival at our ears. Another is the time of arrival of the sound versus path length to our ears. Another is diffraction of sound waves around the head, resulting in intensity and phase changes, the head being a fraction of wavelength at low frequency levels and increasing in size to wavelength multiples at higher frequencies. Another is that at high frequencies, the head acts as a shadow area, and intensity in the direction of the ear towards the source is greater than the ear in the shadow area. For frequencies above about 1200 hertz (cycles per second), the sound shadow cast by the ear lobes is enough to allow the ears to distinguish sounds arriving in front or behind the listener. Finally, binaural localization of a sound source depends on the ability of the ear to detect differences in time of arrival of sound waves at the ears. Lacking this ability of the ears to detect this difference, results in the sound appearing to come from the median plane of the head, i.e., apparently emanating from the center and inside the head of the listener. In stereo listening, this effectively allows the center channel to be monaural.
Commercially available stereophonically recorded program materials include two completely separate channels of program information which are formed, for example, by combining recorded signals from microphones disposed at many locations in the recording studio. When reproduced by a pair of loudspeakers located in a room, such stereophonic program material provides not only directionality, but also, because the sound from each loudspeaker reaches both ears of the listener, either directly or after being reflected from surfaces in the room, the left and right channels are mixed before they reach the listener's ears to provide a panorama of sound.
The manner in which the sound from each loudspeaker mixes with that of the other before reaching the ear of the listener depends upon numerous factors. For example, the position of the listener with respect to the loudspeakers, the frequency of the program material, and the size, shape and contents of the room in which the loudspeakers and listeners are located all contribute to this mixing process. The mixing is not, therefore, merely the addition of a portion of one channel to the other, but instead, involves the complex addition of phase-shifted sounds. A similar mixing process occurs if the listener hears the program material live as it is being recorded, and it is this mixing process which provides true "binaural" listening.
A preponderance of commercially available stereophonically recorded program materials presume that some mixing of the left and right channels will occur when played back through loudspeakers. However, when reproduced through headphones, this complex mixing of the left and right channels does not occur, and instead, the program material in the left channel is coupled directly to the left ear of the listener and the program material of the right channel is coupled directly to the right ear. Although the resulting complete separation of the left and right channels provides a most pleasing listening experience, a "panorama" of sound is difficult to achieve with some recorded materials. For example, when listening to a vocalist accompanied by an orchestra, the vocalist may appear in the center, the brass on the right and the strings on the left. Rather than a continuous blending of these three apparent sources of sound, however, gaps may appear between them. The extent to which this effect is noticed varies greatly depending on the nature of the program material, the type of recording technique used, and the sensitivities of the individual listener.
In the prior art, an old device was devised wherein a pair of spring connected ear pieces with no electrical activity were provided with a flexible tube running between the two ear pieces. The device was intended to be used by abutting one ear piece against a telephone receiver whereby extraneous sounds other than from the telephone receiver were shut out and the sounds were transmitted to the abutting ear piece and from the abutting ear piece to the other ear piece by means of the flexible tube. However, in this old device, the ear pieces had no electrical activity of any kind, to say nothing of not having stereophonic speakers as in the instant invention. Also, instead of having a substantially large acoustical passageway between the wearers' ears as the instant invention does, the passive ear piece merely used a relatively small diameter flexible tube.
Headsets have also been used wherein a tube connected both earphones and also passed in front of the wearer's mouth. The tube had a sound entrance opening in the tube in front of the wearer's mouth and the purpose of the tube is to convey sounds from the wearer's mouth to the wearer's ears so the wearer of the headset can hear himself talk.
The headset was not a stereophonic headset and instead of having a large acoustical passageway between the headset wearer's stereophonic speakers and ears a relatively small diameter tube with a mouth sound entrance opening was used. The purpose of the headset and tube was not to listen to stereophonic sounds, the purpose of the tube being merely to convey the wearer's speech to the wearer's ears.
To more fully appreciate the addition of this invention to the arts a brief discussion of stereophonic sound and this invention follows.
Stereophonic headsets existing today are alike in one important aspect. All transduce the electric signal from the left channel of a stereophonic sound recording or radio source to the left ear only, and the right channel to the right ear only, in the form of compressional sound waves. Although there is a pronounced stereo effect to the listener, that is, different sounds seem to come from different directions, the sound received by the listener is still not as realistic, when compared with the sound from a "live" program, as is possible with my invention.
An examination of how sound from a live program is received at the ears of a listener will make apparent the reason for the lack of realism in the prior apparatuses mentioned above. This invention overcomes the lack of realism and more closely approximates the effect of "live" sound at the ears of the listener. Upon hearing directional sound, that is, sound which is radiated from different distinct sources, the human brain "uses" three components of the sound received to deduce direction, distance and spatial quality. Those three components are (1) intensity differences between the sound at the two ears, (2) the phase difference of the sound wave compared between the two ears (the difference in time of arrival of a sound wave between the two ears) and (3) reflection-reverberation patterns due to the boundaries of the listening environment (all of which is familiar to those skilled in acoustic arts). As an example of a phase shift, at a frequency in the middle of the audible range, about 1,000 Hz, the wavelength of sound is about 13.5 inches. If a sound at this frequency arrives at a listener from directly in front of him, the wave arrives at both ears at the same time, in the same phase, and with the same intensity, since the wave has traversed an equal distance to each ear. But if the wave originates from a direction 45.degree. to the left of center, the wave will arrive at the left ear first, and after a slight bend around the head, it arrives at the right ear about one-half a wavelength later than the left ear, and therefore out of phase compared to the left ear. Due to the direct obstruction of the head in the wave front, the wave must spread around the head, causing both the phase lag and an intensity drop between the ears of the listener. At lower and higher frequencies, the same event occurs, but the phase lag and intensity drop for low frequencies are lessened due to the nature of the longer wavelength. At higher frequencies, the phase lag between the ears may be several frequency periods and the intensity drop quite considerable, due to the nature of the shorter wavelengths. In addition, the reflection of the sound off the walls or boundaries of the environment presents both ears of the listener with an additional complex and delayed system of waves in terms of intensity and phase relations. Due to the sensitivity of the ears and the capability of the brain, the listener deduces the intensity and phase differences of the initial transient sound waves in combination with reflection and reverbation to form an accurate mental cognition of the direction, distance and spatial quality of actual live sound.
It is obvious that all prior stereophonic headsets, due to the restriction of directing all the transduced sound from one sound radiator to one ear only, and all the sound from the other sound radiator to the other ear only, cannot simulate for the ears of the listener the quality of live, spatial sound, because the phase differences and reflection-reverberation patterns that are necessary are not present.